Study On Plastic Deformation And Its Effects On The Microstructure And Properties Of High-entropy Alloy With FCC Structure

In this thesis,two typical face-centered cubic(FCC)high-entropy alloys(HEAs),i.e.,an Al0.3CoCrFeNi HEA sensitive to precipitate at high temperature and a CoCrFeMnNi HEA possessing high thermal stability,were chosen as starting materials.The two FCC HEAs were processed by cold rolling and/or high-pressure torsion(HPT)to different strain values and subsequently the deformed HEAs were annealed.Thus,those plastic deformation mechanisms in FCC HEA and then the microstructure and mechanical properties of FCC HEAs after processing with plastic deformations and subsequent annealing were investigated.The results show that:For Al0.3CoCrFeNi HEA,deformation mechanisms induced by cold rolling and HPT are significantly different.The dominant mechanism during cold rolling transforms from dislocation activity(reduction≤30%)gradually to deformation twinning with an increase in cold-rolling reduction.However,during nanocrystallization of grains induced by HPT to～30nm of average grain size,grain refinement is achieved through twins(including primary and secondary twins),de-twins(including primary and secondary de-twins)and twin boundary(TB)subdivision.Primary de-twin process is ascribed to interaction between primary and secondary twins,while secondary de-twin process forms mainly through partial dislocation emissed from grain boundaries(GBs)or TBs gliding along secondary TBs.TB subdivision process is through dislocation-TB interaction,resulting into a TB transformation into conventional GBs.Competition between twin and de-twin processes leads minimum average spacings of primary and secondary twins to be～2.7nm and～0.9nm,respectively.Nanocrystallization of Al0.3CoCrFeNi HEA produced by HPT has a dramatic influence on annealing-induced phase transformation.For coarse-grained HEA with average grain size of～350pm,increasing temperature induces phase transformation from the FCC phase to an L12 phase and finally to a B2 phase,i.e.,FCC phase→Ll2 phase→B2 phase.By contrast,for nanocrystalline HEA with average grain size of～30nm produced by HPT,annealing-induced phase transformation does not include the L12 formation and the B2 phase precipitates directly from the FCC phase at a lower temperature,resulting into a phase transformation sequence:FCC phase→B2 phase.Processing with HPT and subsequent annealing can dramatically strengthen the Al0.3CoCrFeNi HEA.Vickers hardness value of HPT HEA can reach～530HV,indicating a hardeness increment of～380HV compared to～150HV of ascast HEA.The HEA exhibits a superior HPT hardening than conventional single-phase metallic materials.This should be ascribed to its multiple principal-element effect,which can lead to an enhanced solid-solution hardening and a greater grain refinement in HEA.Meanwhile,the nanocrystalline HEA produced by HPT can be further hardened through annealing-introduced B2 phase.Specially,after 8-revolution HPT and subsequent annealing at 500℃ for 1h,the HEA hardness(～615HV)is approximately 4 times higher than～150HV of as-cast HEA.Processing with cold rolling and subsequent annealing has significant effect on the microtruture and mechanical properties of the Al0.3CoCrFeNi HEA.This HEA can be dramatically hardened via 90%cold rolling,thus exhibiting ultimate tensile strength of～1177MPa and uniform elongation of～7.1%.The cold-rolling HEA can be further strengthened without sacrificing uniform ductility by annealing-induced partial recrystallinzation process,where L12 phase and B2 phase were introduced into residually deformed region and recrystallined region,respectively,and deformation-induced stress should be relaxed.During complete recrystallization process,a HEA with proper grain size and with higher volume fraction of B2 phase can be produced via both decreasing annealing temperature and increasing annealing time,to achieve superior match of strength and ductility.Typically,after 90%cold rolling and subsequent annealing at 800℃ for 10h,the HEA exhibits a strength-elongation product of～36600MPa%,which is approximately two times than～17500MPa%of as-cast HEA.For CoCrFeMnNi HEA,processing with cold rolling and subsequent recrystallization annealing for short time provides an access to grian refinement,to hardening and to GB distribution optimalization.During tensile deformation of CoCrFeMnNi HEA,grain orientation rotations are inhomogenous,but matrix and twins within individual grain have rotations along approximately similar tendency.